Display device and electronic apparatus

ABSTRACT

A display device is provided which offers double-sided display and achieves a thin structure and also which prevents deterioration in display quality of each of front and back display units when an illumination unit such as a backlight is shared by these display units. The display device includes a first display unit having a display surface on the front surface thereof; a second display unit having a display surface on the rear surface thereof; and a common illumination unit interposed between the first display unit and the second display unit, for illuminating both the first display unit and the second display unit with light. A transflector is interposed between the second light-emitting surface of the light-guiding member and the second display unit.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional application of U.S. Ser. No. 10/698,035 filed Oct.30, 2003 which claims priority to Japanese Application JP2002-316081filed Oct. 30, 2002, all of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display device and an electronicapparatus, and in particular, it relates to the structure of a displaydevice suitably mounted on a portable electronic apparatus.

2. Discussion

In general, an electro-optical device such as a liquid crystal displaydevice or an electro-luminescent display device is used as a displaydevice mounted on a portable electronic apparatus. In particular, acellular phone has a compact liquid crystal display device mounted in asmall casing thereof. In recent years, a cellular phone of adouble-sided display type, having compact liquid crystal display devicesmounted on the front and back of a thin displaying portion thereof, iscommercially available on the market. Such a cellular phone has a pairof liquid crystal display devices accommodated in the displaying portionof the casing thereof so as to lie back to back and placed so as to bevisible from both the front and back sides thereof.

Many liquid crystal display devices have a structure in which abacklight serving as an illuminator is disposed behind a liquid crystalpanel. Although a reflective liquid crystal display device having nosuch a backlight is available, its application is limited since itsdisplay is invisible in dark places and at night. Although the liquidcrystal display device equipped with a backlight has a drawback inthickness, with the recent advancements in higher definition and colordisplay of a compact liquid crystal display device, most portableelectronic apparatuses have them mounted thereon. In recent years, therehas emerged a transflective liquid crystal display device equipped witha backlight and able to perform both transmissive display and reflectivedisplay.

Since a portable electronic apparatus such as the above-mentionedcellular phone has become more compact and thinner year by year, a thinliquid crystal display device is strongly desired in accordance withthis trend. In order to meet this requirement, thinner liquid crystalpanels and thinner backlights are being developed.

Unfortunately, the above-mentioned known cellular phone of adouble-sided display type has problems in that, since a pair of liquidcrystal display devices must be accommodated in its casing so as to lieback to back, it is hard to make the casing thinner, and, even when eachof the liquid crystal display devices is made thin, the casing isthicker and heavier than a normal portable phone of a single-sideddisplay type.

One method for solving the above problems lies in that a singlebacklight is shared by the front and back liquid crystal display devicesso as to illuminate a pair of front and back liquid crystal panels.Whereas, in a cellular phone of a double-sided display type, since thedisplay areas of the front and back liquid crystal display devices aregenerally different from each other, when a single backlight is sharedby them as mentioned above, the luminance distribution of a large mainpanel is affected by an illumination action of the backlight applied ona small sub-panel disposed behind the main panel, thereby causing a riskthat a shadow of the sub-panel is reflected in a display image of themain panel and thus its display quality deteriorates.

With this background, the present invention has been made in order tosolve the above problems. Accordingly, it is an object of the presentinvention to provide a display device which offers double-sided displayand achieves a thin structure and which prevents deterioration indisplay quality of both front and back display units when an illuminatorsuch as a backlight is shared by these display units.

SUMMARY OF THE INVENTION

A display device according to the teachings of the present inventionincludes a common illumination unit for illuminating both a firstdisplay unit and a second display unit with light. A transflector isinterposed between the two display units. The transflector reflectslight that previously passed through the first display unit back to thefirst display unit and reflects light that previously passed through thesecond display unit back to the second display unit, the reflected lightbeing used for reflective display. In a transmissive mode, thetransreflector transmits light from the common illumination unit to atleast one of the displays.

With the common illumination unit constructed so as to illuminate boththe first display unit and the second display unit with light, twoseparate illumination units are not required, thereby achieving athinner and lighter device. Also, with the transflector interposedbetween a second light-emitting surface of a light-guiding member of theillumination unit and the second display unit, it is possible to tailorthe amount of light that passes through the transflector to the seconddisplay and the amount of light that is reflected by the transflectorback to the first display. Accordingly, it is possible to place apriority on the illumination state of the first display unit, and hencethe display quality of the first display unit can be improved.

Also, with the above structure, a transflector may also be interposedbetween the first light-emitting surface and the first display unit. Inthis case, the second display unit also obtains an optical effectequivalent to that of the first display unit.

According to aspect of the present invention, the first display islarger than the second display. The display area of the first displayunit may extend so as to two-dimensionally overlap the display area ofthe second display unit and an area lying outside the display area ofthe second display unit. With this structure, since the display area ofthe first display unit extends so as to two-dimensionally overlap notonly the display area of the second display unit but also the areaoutside the display area of the second display unit, light emitted fromthe common illumination unit towards the second display unit can causethe display feature of the first display unit to be optically affecteddue to the fact that the display area of the second display unit ispresent behind the first display unit. Hence, a shadow of the displayarea of the second display unit is sometimes reflected in the displaysurface of the first display unit. However, in the display deviceaccording to the present invention, since the transflector is interposedbetween the second light-emitting surface of the light-guiding memberand the second display unit as described above, the optical affectcaused by the second display unit can be reduced, and thus the displayquality of the first display unit can be improved.

According to a feature of an embodiment of the invention, thetransflector also may have a light-diffusing function. With thisstructure, the light-diffusing function improves the evenness ofillumination light from the illumination unit, thereby furtherpreventing the unevenness of display of the first display unit and thesecond display unit. In particular, when the display device has astructure in which light reflected at the transflector is diffused, theevenness of light in the light-guiding member is improved, and theilluminance distribution on the first display unit is made uniform,thereby further preventing the unevenness of display of the firstdisplay unit.

The transflector may have different optical characteristics between anoverlapping portion that overlaps the display area of the second displayunit and remaining non-overlapping portions of the transflector.Accordingly, an optical difference, for example, a difference inluminance levels, between the display units can be created.

If the transflector is formed so as to have uniform opticalcharacteristics across the entire region, a difference in reflectancesbetween the second display unit and the first display unit can cause ablack shadow or a white shadow of the second display unit to bereflected in the display area of the first display unit. By making theoptical characteristics of the transflector in the overlapping portionand the remaining portions to be different, the foregoing black or whiteshadow can be less noticeable. The term optical characteristics of thetransflector means characteristics such as light reflectance, lighttransmittance, light absorptance, light-diffusing rates, and other lightproperties that can affect the display features of the first displayunit and the second display unit.

In the display device according to the present invention, thetransflector can take many forms. One form is a thin film composed of areflective material and having a thickness allowing light to besubstantially transmitted therethrough. With this structure, since thetransflector is provided with a light transmittivity by adjusting thethickness of a thin film composed of a reflective material, a step suchas patterning can be eliminated, thereby easily making the transflector.A thin metal film can be as the reflective material. In particular,aluminum, an aluminum alloy, silver, a silver alloy, and the like may beused as the reflective material.

Alternatively, transflector is can be composed of a thin reflective filmhaving a plurality of fine apertures dispersed therein such that lightis substantially transmitted therethrough. With this structure, sincethe transflector is provided with a light transmittivity by having thefine apertures dispersed in a thin reflective film, the lighttransmittance is accurately controlled by changing the aperture arearatio of the apertures, for example, by changing the size and thedensity of the apertures.

In another form, the transflector may be formed by a base membercomposed of a light transmissive material and a light-diffusing layerhaving fine particles dispersed in the base member and composed of alight transmissive material having a different refractive index fromthat of the base member. With this structure, the transmittivity and thereflectivity of the transflector can be adjusted by changing adifference in refractive indexes between the base material and the fineparticles and also by changing the size and the density of the fineparticles, and also, the unevenness of display can be prevented by thelight-diffusing effects of the transflector. In particular, with thisstructure, the transmittance of the transflector can be easily madehigher.

In still another form, the transflector may be formed by a base membercomposed of a light transmissive material and a light-diffusing layerhaving fine particles dispersed in the base member and composed of areflective material. With this structure, since light is diffused by thefine particles dispersed in the base member and composed of a reflectivematerial, the transmittivity and the reflectivity of the transflectorcan be adjusted by changing the size and the density of the fineparticles, and also, the unevenness of display can be prevented by thelight-diffusing effects of the transflector. In particular, with thisstructure, the reflectance of the transflector can be easily madehigher.

In the display device according to the teachings of the presentinvention, a light diffuser may be interposed between the firstlight-emitting surface of the light-guiding member and the first displayunit. With this structure, since the light diffuser is interposedbetween the first light-emitting surface and the first display unit, theevenness of the illuminance distribution of the illumination unit can beimproved, thereby further preventing the unevenness of display of thefirst display unit.

The light diffuser may be formed so as to have different opticalcharacteristics between the overlapping portion two-dimensionallyoverlapping the the second display unit and the remainingnon-overlapping portions. With this structure, the unevenness of displayof the first display unit can be further prevented.

The light diffuser may be formed by a base member composed of a lighttransmissive material and a light-diffusing layer having fine particlesdispersed in the base member and composed of a light transmissivematerial having a different refractive index from that of the basemember. Alternatively, the light diffuser may be formed by a base membercomposed of a light transmissive material and a light-diffusing layerhaving fine particles dispersed in the base member and composed of areflective material. Further alternatively, the light diffuser may beformed by a light diffusing layer composed of a light transmissivematerial and having a structure in which fine undulations are formed onthe surface thereof.

Next, an electronic apparatus according to the present inventionpreferably includes any one of the above-mentioned display devices andcontrollers for controlling the display device. Especially, as the aboveelectronic apparatus, a portable electronic apparatus is effective fromthe viewpoint of easily achieving a thin casing thereof. The presentinvention finds particular utility in so-called clam-shell type cellularphones that have a large main display on the inner surface of a foldablepanel, with a smaller display on the exterior of the phone panel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view schematically illustrating the structure of adisplay device according to a first embodiment of the present invention.

FIGS. 2(A) to 2(E) are sectional views schematically illustrating thestructures of a transflector according to the first embodiment.

FIGS. 3(A) and 3(B) are plan views schematically illustrating thestructures of a transflector according to a second embodiment of thepresent invention.

FIGS. 4(A) and 4(B) are plan views schematically illustrating thestructures of a transflector according to a third embodiment of to thepresent invention.

FIGS. 5(A) and 5(B) are sectional views schematically illustrating thestructures of display devices according to a fourth embodiment of to thepresent invention.

FIG. 6 is a structural block diagram illustrating the structure of anexample electronic apparatus.

FIGS. 7(A) and 7(B) are perspective views schematically illustrating thestructure of a cellular phone serving as the example electronicapparatus.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to the accompanying drawings, preferred embodiments ofdisplay devices and an electronic apparatus according to the presentinvention will be described in detail.

First Embodiment

Referring first to FIG. 1, a liquid crystal display device 100 accordingto a first embodiment of the present invention will be described. Theliquid crystal display device 100 includes a first display unit 110, asecond display unit 120, and an illumination unit 130.

The first display unit 110 is a liquid crystal panel and is formed suchthat substrates 111 and 112 composed of glass or plastic are bonded toeach other via a sealant 113 and have liquid crystal 114 interposedtherebetween. The liquid crystal 114 is formed so as to receivepredetermined electrical fields with electrodes formed on the innersurfaces of the substrates 111 and 112. The substrate 112 has apolarizer 115 disposed on the outer surface thereof, that is, on thefront surface side or on the observing side of the first display unit,and the substrate 111 also has the polarizer 116 disposed on the outersurface thereof, that is, on the back surface side.

The second display unit 120 is also a liquid crystal panel and is formedsuch that substrates 121 and 122 composed of glass or plastic are bondedto each other via a sealant 123 and have liquid crystal 124 interposedtherebetween. The liquid crystal 124 is formed so as to receivepredetermined electrical fields with electrodes formed on the innersurfaces of the substrates 121 and 122. The substrate 122 has apolarizer 125 disposed on the outer surface thereof, that is, on theback surface side or the observing side of the second display unit, andthe substrate 121 has a polarizer 126 on the outer surface thereof, thatis, on the front surface side.

The first display unit 110 and the second display unit 120 have a commonillumination unit 130 interposed therebetween. The illumination unit 130serves as a backlight. The illumination unit 130 includes a light source131 formed by, for example, a cold cathode fluorescent tube or a lightemitting diode (LED), and a light guiding plate 132 receiving lightemitted from the light source 131. The light guiding plate 132 iscomposed of a transparent material such as an acrylic resin. In theillumination unit 130, the light guiding plate 132 is a plate disposedso as to be orthogonal to an optical-axis direction of the displaydevice, and the light source 131 is disposed along a side surface of thelight guiding plate 132. The light guiding plate 132 is disposed suchthat a light incident surface 132 a serving as a side surface thereof isopposed to the light source 131, a first light-emitting surface 132 bserving as a front surface thereof, that is, an upper surface thereof inthe figure, faces the first display unit 110, and a secondlight-emitting surface 132 c serving as a back surface thereof in thefigure, faces the second display unit 120.

The light guiding plate 132 is formed so as to output illumination lightfrom the first light-emitting surface 132 b toward the first displayunit 110 and also to output illumination light from the secondlight-emitting surface 132 c toward the second display unit 120 whileallowing light received from the light source 131 to propagate therein.In the light guiding plate 132, light incident on the firstlight-emitting surface 132 b and the second light-emitting surface 132 cat an angle smaller than a critical angle is subjected to totalreflection, and light incident on the same at the critical angle orgreater is outputted outside from the first light-emitting surface 132 band the second light-emitting surface 132 c. Especially, although notshown in the figure, either or both the first light-emitting surface 132b and the second light-emitting surface 132 c may have undulatedsurfaces or scattering layers formed thereon serving as opticaldeflectors for outputting the light introduced from the light source 131along or close to the optical axis directions of the first display unit110 and the second display unit 120.

The second light-emitting surface 132 c of the light guiding plate 132and the second display unit 120 have a transflector 141 interposedtherebetween. The transflector 141 has a structure in which part ofillumination light from the illumination unit is reflected towards thefront surface side and at least part of the remaining illumination lightis transmitted therethrough towards the back surface side. Also, thetransflector can take many forms as will appear and in this embodimentis made from, for example, a thin metal film or a metal film having alarge number of fine apertures dispersed therein. In addition, thetransflector 141 may be bonded to the rear surface of the light guidingplate 132 or may be formed from a sheet or a plate independent of thelight guiding plate 132. In this embodiment, the transflector isinterposed between the second light-emitting surface 132 c of theillumination unit 130 and the second display unit 120, and is formed soas to entirely cover the second light-emitting surface 132 c of thelight guiding plate 132 and has two-dimensionally-uniform opticalcharacteristics.

FIG. 2 illustrates examples of various structures of the foregoingtransflector 141 for this embodiment. In the transflector having theexample structure shown in FIG. 2(A), a reflective thin film 141Bcomposed of a reflective material, for example, metal such as aluminumis formed on the front surface of a transparent substrate 141A composedof glass, a plastic film, or the like. White pigment such as white resinand titanium oxide, metals such as aluminum and silver, and so forth arepreferred as the reflective material. The transflector having thisexample structure may be formed such that the light guiding plate 132 isused instead of the foregoing transparent substrate 141A and that thereflective thin film 141B is formed on the second light-emitting surface132 c. This applies likewise to the transflector having the otherstructures shown in FIGS. 2(B) to 2(E).

By making a thin film from metal such as aluminum or silver so as tohave a thickness of about 10 nm to 50 nm, the reflective thin film 141Bhas an average transmittance of about 30% to 70% in a visible lightrange as its optical characteristic. The above thin film is formed bydeposition, sputtering, laser ablation, or the like. The reflectance andthe transmittance of the transflector having this example structure canbe adjusted by changing the film thickness.

Also, the transflector may be made from a known dielectric multilayerfilm instead of the reflective material as mentioned above.

In the transflector having the example structure shown in FIG. 2(B), areflective thin film 141C composed of a reflective material, forexample, metal such as aluminum is formed on the front surface of thetransparent substrate 141A. White pigment such as white resin andtitanium oxide, metal such as aluminum and silver, and so forth arenamed as the reflective material. The reflective thin film 141C isformed so as to have an average reflectance of about 90% or more as awhole in the visible light range and also to have a large number of fineapertures 141Ca formed therein. The fine apertures 141Ca are dispersedover the entire surface of the reflective thin film 141C. An equivalentaperture diameter of each fine aperture 141Ca, that is, an aperturediameter of a round aperture having the same area as that of the fineaperture, is preferably about 1 μm to 100 μm, and is more preferablyabout 5 μm to 30 μm. Especially, the fine apertures are preferablyformed so as to be smaller than the sizes of pixels of the first andsecond display units and also to be spaced at intervals smaller than thepitches of the pixels. The reflectance and the transmittance of thetransflector having this example structure can be adjusted by changingthe aperture area ratio of the fine apertures 141Ca. The aperture arearatio is determined by the equivalent aperture diameter and the formeddensity of the fine apertures 141Ca.

In the transflector having the example structure shown in FIG. 2(C), alight-diffusing layer 141D basically composed of a light transmissivematerial is formed on the front surface of the transparent substrate141A. The light-diffusing layer 141D includes a transparent substrate141 d 1 composed of acrylic resin or the like and fine particles 141 d 2dispersed in the substrate 141 d 1. The substrate 141 d 1 and the fineparticles 141 d 2 are composed of materials having different refractiveindexes from each other. Particles composed of silica, acrylic resin,and the like are named as the fine particles. The diameters of theparticles are preferably about 1 μm to 10 μm, and more preferably about4 μm to 5 μm.

In the transflector having this example structure, since the particleshaving different refractive indexes from each other are dispersed in thesubstrate, light is scattered or dispersed in a macroscopic view,whereby the transflector having this example structure exhibits opticalcharacteristics reflecting part of the light and transmitting theremaining light therethrough. The reflectance and the transmittance ofthe transflector having this example structure can be adjusted bychanging a difference in refractive indexes between the substrate andthe particles, the size and the distribution density of the particles,and the like.

In the transflector having the example structure shown in FIG. 2(D), alight-diffusing layer 141E basically composed of a light transmissivematerial is formed on the front surface of the transparent substrate141A. The light-diffusing layer 141E includes a transparent substrate141 e 1 composed of acrylic resin or the like and light reflective fineparticles 141 e 2 dispersed in the substrate 141 e 1. The fine particles141 e 2 are composed of a reflective material. White pigment such aswhite resin and titanium oxide, metal such as aluminum and silver, andso forth are preferred as the reflective material. The diameters of theparticles 141 e 2 are preferably about 1 μm to 10 μm, and morepreferably about 2 μm to 3 μm.

In the transflector having this example structure, since the particlescomposed of a reflective material are dispersed in the substrate, lightis scattered or dispersed in a macroscopic view, whereby thetransflector having this example structure exhibits opticalcharacteristics reflecting part of the light and transmitting theremaining light therethrough. The reflectance and the transmittance ofthe transflector having this example structure can be adjusted bychanging the reflectance, the size, and the density of the particles,and the like.

In the transflector having the example structure shown in FIG. 2(E), thereflective thin film 141B, the same as that shown in FIG. 2(a) composedof a reflective material, is formed on the front surface of thetransparent substrate 141A, and a diffusing layer 141F composed of alight transmissive material, preferably of a transparent material, andhaving fine undulations on the surface thereof is additionally formed onthe thin film 141B. The surface undulation of the diffusing layer 141Fhas a depth of about 2 μm to 3 μm formed at an interval of, for example,about 1 μm to 10 μm, preferably about 3 μm to 4 μm. This surfaceundulation is formed by patterning such as photolithography. Forexample, the above-mentioned surface undulation is formed such thatafter application on the reflective thin film 141B, a transparentphotosensitive resin is exposed with a mask pattern having openingsformed with an interval corresponding to the above-mentioned surfaceundulation and is then developed. Meanwhile, a step of additionallyapplying an additional transparent resin on the foregoing developedtransparent resin or heating the developed transparent resin so as to besoftened may be added so as to provide a smooth surface undulation.

According to this example structure, the transflector obtains reflectionand transmission characteristics in accordance with the thickness of thereflective thin film 141B in the same fashion as mentioned above, andalso is provided with a light-diffusing function since light incident onand reflected at the reflective thin film 141B is scattered by thediffusing layer 141F, thereby improving the evenness of the luminancedistributions of the first display unit 110 and the second display unit120.

Referring back to FIG. 1, the light guiding plate 132 and the firstdisplay unit 110 have a light diffuser 142 interposed therebetween. Thelight diffuser 142 is intended to prevent the unevenness of display,that is, the uneven brightness across the display surface of the firstdisplay unit, caused by the structures of the light guiding plate 132and the other components lying on the back surface side thereof, thatis, lying on the lower side in the figure, by appropriately diffusinglight emitted from the light guiding plate 132. The light diffuser 142may have an example structure in which fine particles composed of anacrylic resin or the like and having an example diameter of about 2 μmto 3 μm are dispersed in a base material composed of an acrylic resin orthe like having a different refractive index from that of the fineparticles, or in which fine undulations are provided on the surfacethereof. More particularly, the light diffuser 142 may have the samestructure as those shown in FIG. 2(C) or 2(D), or a structure formed byremoving the reflective thin film 141B from the structure shown in FIG.2(E). The light diffuser 142 may be disposed while being bonded to thefront surface of the light guiding plate 132 or may be formed from asheet or a plate independent of the light guiding plate 132.

Also, a light diffuser 145 is interposed between the light guiding plate132 and the second display unit 120, and is interposed between thetransflector 141 and the second display unit 120 according to thepresent embodiment. The light diffuser 145 has the same structure asthat of the light diffuser 142.

Meanwhile, in the present embodiment, since an overlapping portion,lying in a region two-dimensionally overlapping the display area of thefirst display unit 110 and two-dimensionally overlapping the displayarea of the second display unit 120, and the remaining portion lying inthe region have different optical structures from each other, in orderto improve the evenness of the display surface of the first displayunit, the foregoing light diffuser may be formed such that theoverlapping portion and the remaining portion have different diffusionrates, for example, haze values, from each other.

In the present embodiment, in order to increase the percentage of lightsubstantially contributing to display in each of the first display unit110 and the second display unit 120, light collectors 143 and 144, thatis, prism sheets 143 and 144 are interposed between the illuminationunit 130 and the first display unit 110, and light collectors 146 and147, that is, prism sheets 146 and 147 are interposed between theillumination unit 130 and the second display unit 120. Each lightcollector has a prism surface for refracting light so as to direct thelight along or close to an optical-axis direction of the display device,that is, the vertical direction in the figure. More particularly, theprism surface is formed by a plurality of ribs/convex portions, eachhaving a triangular cross section, juxtaposed with each other in astripe pattern on the surface of the corresponding collector. The lightcollectors 143 and 144 as well as the light collectors 146 and 147 aredisposed such that the foregoing corresponding ribs extend alongdirections substantially perpendicular to each other.

Next, an operation of the display device according to the presentembodiment and having the above-described structure will be described.In the following description, for simplicity of description, it isassumed that both the first display unit 110 and the second display unit120 serving as liquid crystal panels are formed so as to perform displayin a TN-type liquid crystal mode, that the polarizer 115 transmitslinearly polarized light A (not shown) having a plane of vibrationparallel to the plane of the figure, that is, first polarized light, andreflects linearly polarized light B (not shown) having a plane ofvibration orthogonal to the plane of the figure, that is, secondpolarized light, and that the polarizer 116 transmits the linearlypolarized light B, that is, third polarized light, and absorbs thelinearly polarized light A, that is, a fourth polarized light. Inaddition, it is assumed that the polarizer 125 transmits the linearlypolarized light A and absorbs the linearly polarized light B, and thatthe polarizer 126 transmits the linearly polarized light B and absorbsthe linearly polarized light A. That is, although directionalrelationships among the planes of vibration of the first polarized lightto the fourth polarized light are generally arbitrary, in the followingdescription, the first polarized light and the fourth polarized lightexhibit the same polarized state as each other, and the second polarizedlight and the third polarized light exhibit the same polarized state aseach other.

Light emitted from the light source 131 is introduced in the lightguiding plate 132 and is emitted little by little therefrom towards thefront and back surface sides while propagating in the light guidingplate 132. First, illumination light emitted towards the first displayunit 110 passes through the light diffuser 142, becomes the linearlypolarized light B upon passing through the polarizer 116, passes throughthe light collectors 143 and 144, and is then incident on the liquidcrystal 114. When the liquid crystal 114 is in an OFF-state, uponpassing through the liquid crystal 114, the illumination light becomesthe linearly polarized light A due to the optical rotary power of theliquid crystal, passes through the polarizer 115, and is emitted towardsthe front surface side as transmissive light T1. When the liquid crystal114 is in an ON-state, since the illumination light remains as thelinearly polarized light B even when passing through the liquid crystal114, it is absorbed by the polarizer 115.

In the meantime, external light incident on the first display unit 110becomes the linearly polarized light A upon passing through thepolarizer 115 and is incident on the liquid crystal 114. When the liquidcrystal 114 is in an OFF-state, the external light becomes the linearlypolarized light B, is transmitted through the polarizer 116, and entersthe light guiding plate 132, and then, part of this light is reflectedat the transflector 141, becomes the linearly polarized light A uponpassing again through the polarizer 116 and the liquid crystal 114, istransmitted through the polarizer 115, and is emitted as reflectivelight RD1. Also, when the liquid crystal 114 is in an ON-state, sincethe linearly polarized light A of the external light passing through thepolarizer 115 remains as the linearly polarized light A even whenpassing through the liquid crystal 114, it is absorbed by the polarizer116.

Meanwhile, when the foregoing light collectors 143 and 144 are disposed,since external light is scattered upon being incident on these lightcollectors, the reflective light RD1 is not substantially obtained.Accordingly, when it is expected to effectively use the foregoingreflective light RD1, it is preferable that none of these lightcollectors be disposed.

Next, part of illumination light emitted from the illumination unit 130towards the second display unit 120 is transmitted through thetransflector 141, passes through the light diffuser 145, becomes thelinearly polarized light B upon passing through the polarizer 126, andpasses through the liquid crystal 124. When the liquid crystal 124 is inan OFF-state, upon passing through the liquid crystal 124, the linearlypolarized light B becomes the linearly polarized light A, is transmittedthrough the polarizer 125, and is emitted as transmissive light T2towards the back surface side. When the liquid crystal 124 is in anON-state, since the linearly polarized light B remains as it is evenwhen passing through the liquid crystal 124, it is absorbed thepolarizer 125.

In the meantime, for external light incident on the second display unit120, the linearly polarized light A is transmitted through the polarizer125 and is incident on the liquid crystal 124. When the liquid crystal124 is in an OFF-state, the linearly polarized light A becomes thelinearly polarized light B and passes through the polarizer 126 and thelight diffuser 143, and then, part of this light is reflected at thetransflector 141 while other part of the light is introduced in thelight guiding plate 132. The linearly polarized light B reflected at thetransflector 141 passes through the light diffuser 145 and the polarizer126 as it is, becomes the linearly polarized light A upon passing againthrough the liquid crystal 124, is transmitted through the polarizer125, and is emitted as reflective light RD2. When the liquid crystal 124is in an ON-state, since the above-mentioned linearly polarized light Aremains as it is even when passing through the liquid crystal 124, it isabsorbed by the polarizer 126.

Meanwhile, when the foregoing light collectors 146 and 147 are disposed,since external light is scattered upon being incident on these lightcollectors, the reflective light RD2 is not substantially obtained.Accordingly, when it is expected to effectively use the foregoingreflective light RD2, it is preferable that none of these lightcollectors be disposed.

As described above, in the present embodiment, the first display unit110 performs display with the transmissive light T1 and the reflectivelight RD1. Also, the second display unit 120 performs display with thetransmissive light T2 and the reflective light RD2. The presence of thereflective light RD1 and the reflective light RD2 prevents deteriorationin visibility of display in the case of observing an object at a brightplace like outdoors or in the case of cutting an amount of illuminationlight of the illumination unit 130.

In the present embodiment, the illumination unit 130 is formed so as toemit light towards both the first display unit 110 and the seconddisplay unit 120, and in particular, the light guiding plate 132interposed between the first display unit 110 and the second displayunit 120 is shared by them, thereby making the overall structure of thedisplay device 100 thin and light. Also, with the transflector 141disposed as mentioned above, light in the light guiding plate 132 can bedivided towards both the first display unit 110 and the second displayunit 120 respectively disposed on the front and back sides, the evennessof illumination light emitted from the light guiding plate 132 towardseach of the front and back sides can be improved, and also, externallight incident on the first display unit 110 and the second display unit120 can be reflected at the transflector so as to serve as part ofdisplay light.

In particular, with the presence of the transflector 141, theilluminance distribution of the illumination unit 130 on the firstdisplay unit 110 is unlikely to be optically affected by the presence ofthe second display unit 120. With this structure, even when the displayarea of the first display unit 110 is greater than that of the seconddisplay unit 120 as shown in the figure, a shadow of the display area ofthe second display unit 120 is unlikely to be reflected in the displaysurface of the first display unit 110, thereby improving the displayquality of the first display unit 110.

Meanwhile, as mentioned above, although the display areas of the firstand second display units 110 and 120 overlap with each other, in thecase where the one includes the other or in the case where the twodisplay areas do not partially overlap with each other, the unevennessof display becomes generally noticeable. Hence, the above-describedstructure is especially effective in these cases. However, regardless ofthe above-mentioned cases, when a common illuminator is used toilluminate the display units lying on both the front and back sides, theunevenness of the illuminance distribution of the illumination unit isin general likely to occur. Hence, different from the above cases, evenwhen both the display areas have almost the same size as each other andoverlap so as to align two-dimensionally with each other, the structureof the display device according to the present invention is technicallyeffective in order to achieve the evenness of the luminance of each ofthe display units.

Second Embodiment

Referring next to FIG. 3, a display device according to a secondembodiment of the present invention will be described. Since the displaydevice according to the second embodiment basically has the samestructure as that of the display device according to the firstembodiment shown in FIG. 1 except for its transflector, illustrationsand descriptions of the same parts in the second embodiment will beomitted.

As shown in FIG. 3(A), a transflector 241 according to the presentembodiment is interposed between the second light-emitting surface 132 cof the light guiding plate 132 and the second display unit 120 so as toentirely cover a region AR1 two-dimensionally overlapping the displayarea of the first display unit 110. The transflector 241 has anoverlapping portion AR2 which two-dimensionally overlaps the displayarea of the second display unit 120 and which serves as an aperture241A, and in the region AR1, a portion AR3 other than the overlappingportion AR2 serves as a reflector 241R reflecting visible light. Inother words, the transflector 241 is made of reflective material and iscoextensive with the light guiding plate 132, while including anaperture 241A that is aligned with the second display unit 120.

With this structure, the second display unit 120 is illuminated withlight emitted from the light guiding plate 132 and passing through theaperture 241A, thereby easily achieving a bright display of the seconddisplay unit 120. Also, with respect to the first display unit 110,since substantially all light is reflected at the portion AR3, an amountof light contributing to display as a whole can be increased. However,in this case, since the intensity of the illumination light illuminatingthe first display unit 110 decreases in the overlapping portion AR2, itis preferable that the display device have a structure in which theintensity distribution of the illumination light can be modified byadjusting the light-diffusing effect of the light diffuser 142, thelight-emitting distribution of the light guiding plate 132, and thelike.

FIG. 3(B) illustrates a modification to the transflector of the presentembodiment. A transflector 241′ according to the modification is formedsuch that a large number of fine apertures 241A′ are dispersed in thereflective surface of the overlapping portion AR2 and that a portionother than the overlapping portion AR2 has no apertures and serves asthe reflector 241R′ reflecting light. In this modification, the seconddisplay unit 120 is illuminated with light passing through the fineapertures 241A′ formed in the overlapping portion AR2. However, in theoverlapping portion AR2, since light is reflected at the reflectivesurface other than the fine apertures 241A′ towards the first displayunit 110, a difference in brightness of display between two parts of thedisplay surface of the first display unit 110 respectively correspondingto the overlapping portion AR2 and the remaining portion AR3 can bereduced. Meanwhile, also in this case, in order to further prevent theunevenness of display of the first display unit 110, it is preferablethat the display device have a structure in which the intensitydistribution of the illumination light can be modified by adjusting thelight-diffusing effect of the light diffuser 142, the light-emittingdistribution of the light guiding plate 132, and the like.

Third Embodiment

Referring next to FIG. 4, a transflector for a display device accordingto a third embodiment of the present invention will be described. Sincethe display device according to the present embodiment basically has thesame structure as that of the display device according to the firstembodiment shown in FIG. 1 except for its transflector, illustrationsand descriptions of the same parts in the third embodiment will beomitted.

As shown in FIG. 4(A), in the region AR1, a transflector 341 is formedsuch that the overlapping portion AR2 is composed of a transflectivematerial 341A having a predetermined reflectance and transmittance, forexample, both in the range from 30 to 70%. The portion AR3 is composedof a reflective material 341B having a lower transmittance than that ofthe overlapping portion AR2 while having substantially the samereflectance as that of the overlapping portion AR2, for example, in therange from 30 to 70%. It is preferable that the transmittance of thereflective material 341B of the portion AR3 be nearly 0%. Such amaterial is easily obtained, especially from a dielectric multilayerfilm.

With this structure, the overlapping portion AR2 in the region AR1allows illumination light emitted from the illumination unit 130 to bereflected towards the first display unit 110 and to be transmittedtherethrough towards the second display unit 120 at the same time,whereby both display units can be illuminated at the same time. Inaddition, the portion AR3 other than the overlapping portion AR2 doesnot allow unnecessary light to be transmitted therethrough towards theback thereof (to the second display unit 120) because of its lowtransmittance, and also, since the reflectance of the portion AR3 issubstantially the same as that of the overlapping portion AR2, theunevenness of display of the first display unit 110 can be prevented.

According to this example structure, when it is intended to remove theunevenness of the display of the first display unit 110, a light loss ofillumination light due to absorption by the portion AR3 is generated,whereby a light utilization efficiency of the display device as a wholedecreases. In order to improve the light utilization efficiency, it isnecessary to reduce the light loss in the portion AR3 by making thereflectance of the portion AR3 higher relative to that of theoverlapping portion AR2, and it is preferable that the unevenness ofdisplay of the first display unit 110 possibly caused by thisarrangement be modified by changing the structures of the light-guidingplate 132, the light diffuser 142, and so forth.

FIG. 4(B) illustrates a modification of the transflector of the presentembodiment. The entire transflector 341′ is composed of a reflectivematerial. The transflector 341′ has a large number of fine apertures341A′ dispersed in the overlapping portion AR2, and, in the portion AR3the apertures are filled with fine absorbers 341B′ composed of a blackresin or the like. The apertures 341A′ are not filled in portion AR2

The transflector 341′ has a structure in which the fine apertures 341A′in the overlapping portion AR2 allow the second display unit 120 lyingat the back thereof to be illuminated with light, and also the fineabsorbers 341B′ formed in the remaining portion AR3 reduce a differencein reflectances between the overlapping portion AR2 and the portion AR3.With this structure, the unevenness of display of the first display unit110 can be prevented.

Fourth Embodiment

Referring next to FIG. 5, a display device according to a fourthembodiment of the present invention will be described. The displaydevice according to the present embodiment has a structure in which anyone of the display devices according to the foregoing embodiments isaccommodated in a casing composed of a synthetic resin, or the like.Accordingly, since the display device of the present embodimentbasically has the same structure as that of the display device accordingto the first embodiment shown in FIG. 1 except for its structuralfeature of accommodation with the casing, illustrations of the sameparts in the present embodiment will be simplified and descriptions ofthe same parts will be omitted.

As shown in FIG. 5(A), a display device 100′ includes the first displayunit 110, the second display unit 120, the illumination unit 130, andthe other foregoing components (not shown), which are the same as thoseof each of the display devices according to the foregoing embodiments,and these components are accommodated in a casing 150. The casing 150 iscomposed of, for example, a white synthetic resin. Making the casing 150from a white material is preferable from the viewpoint of improving autilization efficiency of illumination light of the illumination unit130 since this casing reflects light leaked from the light guiding plate132 and returns again to the light guiding plate 132.

In the present embodiment, an area not having the second display unit120 disposed therein, on the front surface of the second light-emittingsurface of the light guiding plate 132 of the illumination unit 130,that is, the foregoing portion AR3, has an optical sheet 151 havingpredetermined optical characteristics and disposed therein. When viewedfrom the first display unit 110, the optical sheet 151 is composed of amaterial having substantially the same reflectance as that of the seconddisplay unit 120. With this structure, even when the transflector 141has entirely uniform optical characteristics, the unevenness of displayof the display surface of the first display unit can be decreased.

FIG. 5(B) illustrates a modification of the present embodiment. In adisplay device 100″ according to this modification, the first displayunit 110, the second display unit 120, the illumination unit 130, andthe other components (not shown), which are the same as those of each ofthe display devices according to the foregoing embodiments, areaccommodated in a casing 160. When viewed from the first display unit110, the casing 160 is composed of a material exhibiting substantiallythe same reflectance as that of the second display unit 120. With thisstructure, even when the foregoing transflector 141 has entirely uniformoptical characteristics, the unevenness of display of the displaysurface of the first display unit can be decreased.

Thus, in accordance with the teachings of this embodiment, the opticalsheet 151 and the casing 160 are formed from materials such that, whenobserved from the first display unit 110, the illuminance distributionswhere the second display unit 120 and the optical sheet 151 (or thecasing 160) are disposed are made uniform regardless of the structuresof the transflector 141 and the light guiding plate 132.

Fifth Embodiment

Referring next to FIGS. 6 and 7, an electronic apparatus, including thedisplay device 100 according to the first embodiment, will be described.As shown in FIG. 6, the electronic apparatus according to the presentembodiment includes a controller 1100 for controlling the first displayunit 110, that is, the liquid crystal display panel 110, and acontroller 1200 for controlling the second display unit 120, that is,the liquid crystal display panel 120. The controllers 1100 and 1200 arecontrolled by a central controller 1000 disposed in the electronicapparatus and formed by a microcomputer and so forth.

The first and second display units 110 and 120 are respectivelyconnected to drive circuits 110D and 120D including semiconductor ICsand the like, which are mounted on the corresponding panels or connectedto the corresponding panels via respective wiring members, and thesedrive circuits 110D and 120D are respectively connected to thecontrollers 1100 and 1200. The controllers 1100 and 1200 respectivelyinclude display-information output sources 1110 and 1210,display-information process circuits 1120 and 1220, power supplycircuits 1130 and 1230, and timing generators 1140 and 1240.

Each of the display-information output sources 1110 and 1210 includes amemory such as a read only memory (ROM) and/or a random access memory(RAM), a storage unit such as a magnetic storage disk and/or an opticalstorage disk, and a tuning circuit for outputting a tuned digital imagesignal. Also, in response to a variety of clock signals generated by thetiming generators 1140 and 1240, the display-information output sources1110 and 1210 supply display information, in the form of an image signalaccording to a predetermined format or the like, to thedisplay-information process circuits 1120 and 1220, respectively.

Each of the display-information process circuits 1120 and 1220 includesa variety of known circuits such as a serial-parallel conversioncircuit, an amplification and reversion circuit, a rotation circuit, agamma correction circuit, and a clamp circuit, processes the inputdisplay information, and supplies the processed image informationtogether with a clock signal CLK to the corresponding drive circuit.Each of the drive circuits 110D and 120D includes a scan-line drivecircuit, a data-line drive circuit, and a testing circuit. Also, each ofthe power supply circuits 1130 and 1230 supplies a predetermined voltageto each of the above described corresponding components.

The central controller 1000 sends original data for illuminationlights-on and -off commands and display information, and the like, ifneeded, to the display-information output sources 1110 and 1210 of thecorresponding controllers 1100 and 1200, controls thedisplay-information output sources 1110 and 1210 so as to output displayinformation in response to the original data and the like, and alsocontrols the first display unit 110 and the second display unit 120 soas to display required display images via the controllers 1100 and 1200and the drive circuits 110D and 120D, respectively. Also, the centralcontroller 1000 is formed so as to control the illumination lights-onand -off, and the like of the light source 131.

FIG. 7 illustrates a cellular phone 2000 serving as an example of theelectronic apparatus according to the present invention. The cellularphone 2000 includes a main body 2001 having a variety of buttonsdisposed thereon and a microphone housed therein, and a display unit2002 having a display screen and an antenna mounted thereon and aspeaker housed therein, and the main body 2001 and the display unitcasing 2002 are constructed so as to be mutually foldable. The displayunit casing 2002 has both the first and second display units 110, 120housed therein, with the display screen of the first display unit 110being disposed on the inner surface thereof so as to be visible and thedisplay screen of the second display unit 120 being disposed on theouter surface thereof so as to be visible.

In the present embodiment, as shown in FIG. 7(A), when the display unitcasing 2002 is opened by separating or folding it from the main body2001, in response to a command from the central controller 1000, thefirst display unit 110 is illuminated, and a predetermined image isdisplayed. Also, as shown in FIG. 7(B), when the display unit casing2002 is folded onto the main body 2001, the first display unit 110 isillumination lights-off, the second display unit 120 is illuminatedinstead of the above illumination lights-off, and a predetermined imageis displayed.

In the present embodiment, the display device 100 having a thinstructure as mentioned above allows the display unit casing 2002 to havea thin structure, to have a simple internal structure, and to be easilyassembled. Meanwhile, the electronic apparatus according to the presentembodiment may also have any one of the display devices in the second tofourth embodiments mounted thereon.

Meanwhile, the electro-optical device and the electronic apparatusaccording to the present invention are not limited to the foregoingexample illustrations, and those skilled in the art will appreciate thata variety of modifications can be possible without departing from thespirit of the present invention. For example, although a liquid crystaldisplay panel is used as an electro-optical panel in each of theforegoing embodiments, any one of a variety of electro-optical panelssuch as an organic electro-luminescence panel, a plasma display panel,and a field-emission display panel may also be used as theelectro-optical panel according to the present invention. Also, althoughthe liquid crystal display panel of a passive matrix type is basicallyillustrated in the foregoing embodiments, the present invention islikewise applicable to that of an active matrix type.

The entire disclosure of Japanese Patent Application No. 2002-316081filed Oct. 30, 2002 is incorporated by reference.

1. A display device comprising: a first display unit having a displaysurface on a front surface thereof; a second display unit having adisplay surface on a rear surface thereof; a common illumination unitinterposed between the first display unit and the second display unitilluminating both the first display unit and the second display unitwith light; and a transflector interposed between the light-guidingmember and the second display unit, the transflector being made ofdielectric multilayer film having predetermined optical transmissive andreflective characteristics.
 2. The display device of claim 1 whereinsaid predetermined optical transmissive and reflective characteristicsare chosen to favor the illuminance of the first display unit over thesecond display by determining the amount of light that is reflected backto the first display that had previously passed through the firstdisplay unit, by determining the amount of light that is reflected backto the second display that had previously passed through the seconddisplay unit, and by determining the amount of light transmitted to thesecond display from the illumination unit.
 3. The display device ofclaim 1, wherein the first display unit is larger than the seconddisplay unit, two-dimensionally overlaps the second display unit andcovers an extended area beyond the second display unit.
 4. The displaydevice of claim 1, which further comprises a first light diffuserbetween the illumination unit and the first display unit, and a secondlight diffuser between the illumination unit and the second displayunit.
 5. The display device of claim 3 which further comprises: a hingedcasing for a cellular phone and containing the display units; said firstdisplay unit facing an inner surface of the casing and the seconddisplay unit facing an outer surface of the casing.
 6. The displaydevice of claim 5 which further comprises: an optical sheet surroundingthe second display unit and having the substantially the samereflectance as the second display unit to minimize shadow effects on thefirst display unit.
 7. The display device of claim 5 wherein portions ofthe casing surrounding the second display are made from a materialhaving substantially the same reflectance as the second display unit tominimize shadow effects on the first display unit.
 8. The display deviceof claim 6 wherein the transflector is substantially coextensive withthe first display unit and overlaps the second display unit, thetransflector having substantially the same optical characteristicsthroughout its area.
 9. The display device of claim 7 wherein thetransflector is substantially coextensive with the first display unitand overlaps the second display unit, the transflector havingsubstantially the same optical characteristics throughout its area. 10.The display device of claim 1 wherein the common illumination unitincludes a light guide member having a first light emitting surfacefacing the first display unit and a second light emitting surface facingin the opposite direction of the first light emitting surface
 11. Thedisplay device of claim 10 further comprising: a first light collectordisposed between the first display unit and the first light emittingsurface of the light guide member; and a second light collector disposedbetween the second display unit and the second light emitting surface ofthe light guide member, the second light collector being smaller thanthe first light collector.
 12. The display device of claim 4 wherein thesecond light diffuser is smaller than the first light diffuser.
 13. Adisplay device, comprising: a first display unit having a viewed side; asecond display unit having a viewed side facing in an opposite directionthan the viewed side of the first display unit; an illumination unitdisposed between the first display unit and the second display unit andilluminating light onto both the first display unit and the seconddisplay unit, a light diffuser disposed between the first display unitand the first light emitting surface of the illumination unit; and asheet including a ribbed portion disposed between the second displayunit and the illumination unit, the sheet reflecting a portion ofincident light and transmitting a portion of incident light, the sheetbeing in an overlapping condition with the second display unit in planview.
 14. A display device, comprising: a first display unit having aviewed side; a second display unit having a viewed side facing in anopposite direction than the viewed side of the first display unit, thesecond display unit being smaller than the first display unit; anillumination unit disposed between the first display unit and the seconddisplay unit and illuminating light onto both the first display unit andthe second display unit, the illumination unit including a light guidemember having a first light emitting surface facing the first displayunit and a second light emitting surface facing in the oppositedirection of the first light emitting surface; a first light collectordisposed between the first display unit and the first light emittingsurface of the light guide member; and a second light collector disposedbetween the second display unit and the second light emitting surface ofthe light guide member, the second light collector being smaller thanthe first light collector.
 15. The display device of claim 14, furthercomprising a transflector disposed between the second light emittingsurface of the light guide member and the second display unit, thetransflector reflecting a portion of incident light and transmitting aportion of incident light.
 16. A display device, comprising: a firstdisplay unit having a viewed side; a second display unit having a viewedside facing in an opposite direction than the viewed side of the firstdisplay unit, the second display unit being smaller than the firstdisplay unit; an illumination unit disposed between the first displayunit and the second display unit and illuminating light onto both thefirst display unit and the second display unit, the illumination unitincluding a light guide member having a first light emitting surfacefacing the first display unit and a second light emitting surface facingin the opposite direction of the first light emitting surface; a firstlight diffuser disposed between the first display unit and the firstlight emitting surface of the illumination unit; and a second lightdiffuser disposed between the second display unit and the second lightemitting surface of the illumination unit, the second light diffuserbeing smaller than the first light diffuser.
 17. The display device ofclaim 16, further comprising a transflector disposed between the secondlight emitting surface of the light guide member and the second displayunit, the transflector reflecting a portion of incident light andtransmitting a portion of incident light.
 18. The display device ofclaim 16, wherein the first display unit overlaps in plan view thesecond display unit as well as a region outside the second display unit.19. A display device comprising: a first display unit having a displaysurface on a front surface thereof; a second display unit having adisplay surface on a rear surface thereof; a common illumination unitinterposed between the first display unit and the second display unitilluminating both the first display unit and the second display unitwith light; a first light collector disposed between the first displayunit and the light guide member; a second light collector disposedbetween the second display unit and the light guide member; and saidfirst and second light collectors being prism sheets.
 20. The displaydevice of claim 19 wherein one surface of the first light collector hasribs extending in one direction and wherein an opposite surface of thefirst light collector has ribs extending in an opposite direction. 21.The display device of claim 19 wherein one surface of the second lightcollector has ribs extending in one direction and wherein an oppositesurface of the second light collector has ribs extending in an oppositedirection.
 22. The display device of claim 19 further comprising: atransflector interposed between the common illumination unit and thesecond display unit.
 23. The display device of claim 22 wherein thetransflector is made of dielectric multilayer film having predeterminedoptical transmissive and reflective characteristics.
 24. The displaydevice of claim 23 wherein said predetermined optical transmissive andreflective characteristics are chosen to favor the illuminance of thefirst display unit over the second display by determining the amount oflight that is reflected back to the first display that had previouslypassed through the first display unit, by determining the amount oflight that is reflected back to the second display that had previouslypassed through the second display unit, and by determining the amount oflight transmitted to the second display from the illumination unit. 25.The display device of claim 24, wherein the first display unit is largerthan the second display unit, two-dimensionally overlaps the seconddisplay unit and covers an extended area beyond the second display unit.26. The display device of claim 19, which further comprises a firstlight diffuser between the illumination unit and the first display unit,and a second light diffuser between the illumination unit and the seconddisplay unit.
 27. The display device of claim 19 which furthercomprises: a hinged casing for a cellular phone and containing thedisplay units; said first display unit facing an inner surface of thecasing and the second display unit facing an outer surface of thecasing.
 28. The display device of claim 19 which further comprises: anoptical sheet surrounding the second display unit and having thesubstantially the same reflectance as the second display unit tominimize shadow effects on the first display unit.
 29. The displaydevice of claim 19 wherein portions of the casing surrounding the seconddisplay are made from a material having substantially the samereflectance as the second display unit to minimize shadow effects on thefirst display unit.
 30. The display device of claim 24 wherein thetransflector is substantially coextensive with the first display unitand overlaps the second display unit, the transflector havingsubstantially the same optical characteristics throughout its area. 31.The display device of claim 19 wherein the common illumination unitincludes a light guide member having a first light emitting surfacefacing the first display unit and a second light emitting surface facingin the opposite direction of the first light emitting surface, andwherein the first light collector is disposed between the first lightemitting surface and the first display unit, and wherein the secondlight collector is disposed between the second light emitting surfaceand the second display unit.
 32. The display device of claim 19 whereinthe second light collector is smaller than the first light collector.33. The display device of claim 26 wherein the second light diffuser issmaller than the first light diffuser.
 34. A display device, comprising:a first display unit having a viewed side; a second display unit havinga viewed side facing in an opposite direction than the viewed side ofthe first display unit, the second display unit being smaller than thefirst display unit; an illumination unit disposed between the firstdisplay unit and the second display unit and illuminating light ontoboth the first display unit and the second display unit, theillumination unit including a light guide member having a first lightemitting surface facing the first display unit and a second lightemitting surface facing in the opposite direction of the first lightemitting surface, at least one of the first light emitting surface andthe second light emitting surface having an undulated surface; a firstlight collector disposed between the first display unit and the firstlight emitting surface of the light guide member, the first lightcollector including a first prism sheet with ribs extending in a firstdirection and a second prism sheet with ribs extending in a seconddirection substantially perpendicular to the first direction; and asecond light collector disposed between the second display unit and thesecond light emitting surface of the light guide member, the secondlight collector including a first prism sheet with ribs extending in athird direction and a second prism sheet with ribs extending in a fourthdirection substantially perpendicular to the third direction.
 35. Thedisplay device of claim 34 wherein the ribs are triangular-shaped. 36.The display device of claim 34 further comprising a transflectordisposed between the second light emitting surface of the light guidemember and the second display unit, the transflector reflecting aportion of incident light and transmitting a portion of incident light.37. The display device of claim 34, wherein the first display unitoverlaps in plan view the second display unit as well as a regionoutside the second display unit.
 38. The display device of claim 37which further comprises: an optical sheet surrounding the second displayunit and having the substantially the same reflectance as the seconddisplay unit to minimize shadow effects on the first display unit. 39.The display device of claim 34 wherein the second light collector issmaller than the first light collector.
 40. The display device of claim36 wherein the transflector is made of dielectric multilayer film havingpredetermined optical transmissive and reflective characteristics.